1. Field of the Invention
The present invention relates to a polymeric fluorescent substance and an organic electroluminescent element using the polymeric fluorescent substance.
2. Background Art
Electroluminescent elements (“electroluminescent” being hereinafter often abbreviated to “EL”) are luminescent elements utilizing electroluminescence. The construction of electroluminescent elements is basically anode/organic luminescent layer/cathode, and electroluminescent elements, in which a hole injection transport layer or an electron injection layer is properly added to the above basic construction, are known in the art. Among organic EL elements are those using a polymeric fluorescent substance as a luminescent material. For example, a polymeric fluorescent substance of a poly-p-phenylenevinylene derivative is proposed in J. H. Burroughes et al., Nature, vol. 347, P. 539 (1990) and D. Braun et al., Appl. Phys. Lett., vol. 58, P. 1982 (1991).
However, it is difficult to provide blue luminescence from the poly-p-phenylenevinylene derivative. On the other hand, a polyfluorene polymer is proposed in M. Fukuda et al., Jpn. J. Appl. Phys., vol. 28, p. L1433 (1989) and Y. Ohmori et al., Jpn. J. Appl. Phys., vol. 30, p. L1941 (1991). The polyfluorene is thermally and chemically stable and exhibits very strong fluorescence in a solution or solid state. Further, the polyfluorene is very advantageous in that the introduction of a substituent such as a long alkyl chain renders the polyfluorene soluble in organic solvents and thus can provide excellent film forming properties and can relatively easily realize blue luminescence.
However, it has been often pointed out π-conjugated polymers such as poly-p-phenylenevinylene and polyfluorene have liquid crystallinity derived from the molecular structure and, upon heating or the application of an electric field, molecular chains per se associate to form excimer which shifts the luminescence wavelength to a longer wavelength or deteriorates luminescence efficiency.
From the viewpoint of suppressing such excimer luminescence, research and development for enhancing the thermal stability have been made. It is estimated that polymer chains having a high molecular weight have small molecular chain mobility due to such a state that, in high-molecular weight polymer chains, yarn spheres have been entangled with each other while polymer chains having a low molecular weight have large molecular chain mobility and, thus, upon heating or application of an electric field, are unfavorably moved and associate.
The excimer formation is significant particularly in blue luminescent elements. To overcome this drawback, an element comprising a luminescent layer using a polymer, from which a low molecular weight component has been removed by GPC (gel permeation chromatography), is proposed in K.-H. Weinfurtnaer et al., Appl. Phys. Lett., vol. 76, p. 2502 (2000).
In this element, however, the luminescence efficiency at 100 cd/m2 is low and 1.55 cd/A. Thus, there is still a need for a polymeric organic EL element possessing excellent blue luminescence, service life, luminescence efficiency, and thermal stability.